The production of polyurethane coatings on PVC is already known in principle. Methods of coating PVC using one-component or two-component polyurethane systems dissolved in organic solvents have been described, for example, in U.S. Pat. Nos. 3,262,805; 4,017,493 and 4,045,600. The use of coatings containing solvents, however, has certain practical disadvantages, particularly the fact that for economical and ecological reasons it is necessary to recover the organic solvent after the coating process. There is therefore a general demand for solvent-free coating compositions.
It is known from German Published Patent Application 1,769,387 (British Pat. No. 1,191,260) to bond PVC with aqueous polyurethane dispersions, but these dispersions have not been successful as top coats or finishes on PVC. This is because it has not hitherto been possible to provide products which would both adhere firmly to PVC and meet the high quality standards of the coating industry for top coats and finishes (e.g. high softening point; high mechanical strength; stability to ageing by hydrolysis; low swelling in plasticizers; blocking against migration of the plasticizers in PVC; and pleasant, dry handle).
It has now surprisingly been found that coating compositions which fulfill the above-mentioned requirements can be obtained from aqueous polyurethane dispersions by mixing two different polyurethane dispersions each of which must fulfill different chemico-physical criteria.